There are described in the prior art carboxylic acid amides, namely dicarboxylic acid diamides, and some tricarboxylic acid triamides and tetracarboxylic acid tetraamides, which have the ability of forming lipophilic complexes with several kinds of cations. It is furthermore also known that these carboxylic acid amides may be used as ion-sensitive components in ion-sensitive parts for the determination of cations in sample solutions.
For a long time, investigations were made to develop carboxylic acid amides which are sufficiently selective for magnesium ions over alkali metal ions and calcium ions, to permit the determination of magnesium ions in sample solutions containing sodium ions, calcium ions and potassium ions. The known lipophilic complex-forming agents for magnesium had at best only about the same selectivity for magnesium ions as for calcium ions; or the determination of magnesium ions was possible only in sample solutions having a basic pH. Higher concentrations of protons interfered with magnesium ion determination.
It is the object of the present invention to provide carboxylic acid amides which have a sufficiently high selectivity for magnesium ions over other alkali metal ions and alkaline earth metal ions so that the determination of magnesium ions may be performed in sample solutions of biological origin, e.g., body fluids. It is a further object to provide carboxylic acid amides which also have the ability to form lipophilic complexes with magnesium ions in the presence of higher activities of H.sub.3 O+ ions. This permits determination of magnesium ions in sample solutions which have a neutral pH or even slightly acidic pH, e.g., a pH value in the range of 5.5 to 7.5.